Coupled coaxial cavity travelingwave tube



Feb. 23, 1965 P. DONG ETAL 3,171,054

COUPLED COAXIAL CAVITY TRAVELING-WAVE TUBE Filed Aug. 20, 1962 2 Sheets-Sheet l llr-ll- Ama/razr.

Feb. 23, 1965 P. DoNG ETAL COUPLED COAXIAL CAVITY TRAVELING-WAVE TUBE 2 Sheets-Sheet 2 Filed Aug. 20, 1962 ,L l a i www @f M M M M 3f 5 f 4 ,K i 5 4 A ma ww! United States Patent O This invention relates generally to microwave devices, and more particularly relates to a traveling-wave tube having a coupled coaxial cavity slow-wave circuit with improved frequency-phase characteristics.

ln traveling-wave tubes a stream of electrons is caused to interact with a propagating electromagnetic wave in a manner which amplies the electromagnetic energy. In order to achieve such interaction, the electromagnetic wave is propagated along a slow-wave structure, such as a conductive helix wound about the path of the electron stream or a folded waveguide type of structure in which a waveguide is effectively wound back and forth across the path of the electrons. The slow-wave structure provides a path of propagation for the electromagnetic wave which is considerably longer than the axial length of the structure, and hence the traveling-wave may be made to efectively propagate at nearly the velocity of the electron stream. The interactions between the electrons in the stream and the traveling-Wave cause velocity modulations and bunching of electrons in the stream. The result may then be a transfer of energy from the electron beam to the wave traveling along the slow-wave structure.

As the traveling-wave tube art developed, attempts have been made to design tube capable of handling higher and higher amounts of average power and yet still operate at relatively low voltage levels. Some of the prior art traveling-wave tubes, such as those having helix slowwave circuits, are nevertheless limited in their power handling capabilities. On the other hand, while tubes which are capable of handling larger amounts of average power have been developed, the impedance which the slowwave circuits of these tubes present to a high current density electron beam is quite low. Moreover, the frequency-phase characteristics of these tubes are too dispersive in the high impedance region to achieve maximum gain and eiiiciency, and at the same time, are not Sufiiciently dispersive at the upper cutoff frequency to eliminate oscillation problems.

Accordingly, it is an object of the present invention to provide a traveling-wave tube capable of dissipating a large amount of average power while still operating at a relatively low voltage level. v

lt is a further object of the presentinvention to provide a traveling-wave tube which not only has an improved impedance-bandwidth product, but which also possesses a specially shaped frequency-phase characteristic to reduce any tendency for oscillations at the upper cutoff frequency of the tubes passband.

lt is a still further object of the present invention to provide a coupled coaxial cavity slow-wave circuit for a high power, hollow beam traveling-wave tube which facilitates essentially uniform electric fields in the Wavebeam interaction regions.

in accordance with the objects outlined above, the present invention provides a traveling-wave tube including a tubular envelope, means at one end of the envelope for launching an annular electron stream along a predetermined axial path within the envelope, and a plurality of walls mounted in the envelope at spaced points therealong and extending perpendicular to the envelope to define therewith a plurality of cavities. The cavity walls dene aligned apertures which provide a passage for the annular electron stream, and preselected ones of the 3,171,054 Patented Feb, 23, 1965 walls each define a plurality of outer coupling apertures in regions radially outwardly of its electron stream aperture and a plurality of inner coupling apertures in regions radially inwardly of its electron stream aperture. The coupling apertures are arranged to provide a slow-wave structure for propagating an electromagnetic wave in a manner which supports energy exchange between the annular electron stream and the electromagnetic wave, and which slow-wave structure possesses improved frequencyphase characteristics for the propagated electromagnetic waves.

The exact nature of the invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification describing a preferred embodiment of the invention as illustrated in the accompanying drawings in which:

FIG. l is a schematic view, primarily in longitudinal section, illustrating a traveling-wave tube constructed in accordance with the present invention;

FlG. 2 is a cross-sectional view taken along line 2--2 of FIG. l;

FIG. 3 is a cross-sectional view taken along line 3 3 f FIG. l; and

PEG. 4 is an wgraph illustrating the manner in which the slow-wave circuit of the present invention provides improved frequency-phase characteristics.

Referring with more particularity to the drawings, in FIG. l there is illustrated a traveling-wave tube designated generally by the reference numeral lil and having a coupled coaxial cavity slow-wave structure 12 for propagating microwave energy along and about a hollow electron beam traversing the length of the slow-wave structure 12.

At one end of the slow-wave structure 12 there is disposed a conventional hollow beam generating electron gun 14 which may include an annular electron emissive cathode 16 and tilamentary heater 18. The emitted electrons are shaped'and focused into an annular, or hollow,

beam by a focusing electrode Ztl having inner and outer surfaces of revolution disposed aboutthe longitudinal axis of the tube 16 at acute angles therewith. An accelerating anode 22, having an annular aperture aligned with the annular cathode 16, is provided beyond the focusing electrode 20 for accelerating the beam electrons to a high velocity. Voltage sources 26 and 24 provide operating voltages for the heater 18 and the electrodes 16, 26 and 22.

A collector arrangement 30 is disposed at the opposite end Yof the slow-wave structure 12 to intercept the stream of electrons and dissipate their kinetic energy. The collector arrangement comprises a first cylindrical portion 32 adjacent the end of the slow-wave structure and provided with an annular electron-intercepting groove 34 aligned with the electron beam containing region. A second cylindrical portion 36 of the collector 30 is disposed more remote from the slow-wave structure 12, and this portion is provided with annular metallic cooling tins 38 projecting radially outwardly of the portion 36 for dissipating the heat produced by the intercepted stream electrons. A source of potential 4% is connected to the collector 39 to bias the collector V3l) positive to prevent secondarily emitted electrons from the slow-wave structure 12 from returning to the interaction region, thereby reducingnoise and other interference.

An input waveguide 44 is connected to the input end .of slow-wave structure 12 which, although illustrated as the electron gun end in FIG. l, may be the collector end if a backward wave device is desired. The waveguide 44 is connected `to external circuitry (not shown) by a coupling liange 46, which may include a microwave window =to enable a pressure differential to exist between the exterior and the evacuated interior of the traveling-wave tube 10. Similarly, an output waveguide 48 is connected to the opposite end of the slow-wave structure 12. A coupling flange 50, which may be similar to the flange 46 described above, is provided at the end of the waveguide 48 for coupling to external circuitry (not shown). A solenoid 52, energized by a source of potential 54, is disposed concentrically about and substantially coextensive with the slow-wave structure 12 for providing a longitudinal focusing magnetic field which constrains the electron beam to flow along an annular axial path toward the collector 30.

Referring now .to FIGS. 1-3, the coupled coaxial cavity slow-wave structure 12 will now be described in more detail. The slow-wave structure 12 comprises a tubular waveguide 60, which serves as an envelope for the slowwave structure 12, and an electrically conductive cylindrical rod 62 coaxially mounted within the envelope 60. The waveguide 60 and rod 62 are maintained at the same electrical potential, e.g. ground potential, and are electrically insulated from the collector 32 by means of insulating ring 58 and stub 59, respectively. Annular electrically conductive disks, or plates, 64 are mounted within the envelope 60 at spaced points along the envelope 60. Each plate 64 extends radially inwardly from the envelope 60 in a plane perpendicular .to the longitudinal axis of the envelope and defines at its innermost portion an annular rim, or flange, 65 extending parallel to the envelope 6G and beyond the plane of the plate 64 in both directions along the axis of the tube 10. Inner annular electrically conductive plates, or disks, 66 are mounted on and project radially outwardly from the rod 62 in the same planes as the outer plates 64. Each plate 66 denes an annular rim, or flange, 67 at its outermost region, with each flange 67 being parallel to and coextensive with the flange 65 on the associated outer disk 64. Each pair of coplanar plates 64 and 66 functions as an interaction cavity-defining partition, or wall, along the envelope 60,

Vwith the rims 65 and 67 providing an annular passageway 68 through the wall to accommodate the hollow electron beam. Thus, the plates 64 and 66, together with portions of ythe rod 62 and the inner surface of the envelope 60 define a series of annular interaction cells, or cavities, 69.

The respective interaction cavities 69 are interconnected by means of coupling holes in the disks 64 and 66 to permit the transfer of microwave energy from cell to cell along .the slow-wave structure 12. Thus, each outer annular disk 64 is provided with four substantially kidney-shaped coupling holes 70a, b, c, d equally spaced circumferentially around the disk 64, while four smaller substantially kidney-shaped and equally circumferentially spaced coupling holes 72a, b, c, d are provided in each inner annular disk 66. However, the coupling holes 72a, b, c, d in each inner disk 66 are angularly displaced by 45 with respect to the coupling holes 70a, b, c, d in the coplanar outer disk 64 so that the center of each inner coupling hole 72a, b, c, d lies angularly midway between the centers of the two nearest outer coupling holes 70a, b, c, d. Moreover, the outer coupling holes 70a, b, c, d in disk 64 are angularly displaced by 45 with respect to the outer coupling holes in its two neighboring disks, i.e. the nearest disks on each side, lengthwise along the slowwave structure 12, and similanly for the inner coupling holes 72a, b, c, d in inner disk 66. v

In 4the operation of the traveling-wave tube 10, electromagnetic waves launched onto the slow-wave structure 12 via the input waveguide 44 propagate from cavity to cavity along the slow-wave structure 12 by means of coupling holes 70a, b,'c, d and 72a, b, c, d. On account of the staggered relation of the coupling holes, the electromagnetic waves are propagated axially alongfthe slowwave structure 12 at a velocity less than the velocity of light. A propagating electromagnetic wave which traverses a cavity 69 between an inner and an outer coupling hole will cross the annular electron beam in interaction region 74 of the cavity 69, and energy exchange bel, tween the electron beam and the electromagnetic wave then occurs, resulting in amplification of the electromagnetic wave. After Ibeing amplified by interaction with the electron beam in the successive interaction regions 74 of the slow-wave structuren, the output electromagnetic wavc is removed from the slow-wave structure 12 via output waveguide 48. It is to be pointed out that the width of the frequency ranges of waves which will propagate along the slow-wave structure 12 is determined by the size and location of the coupling holes 70a, b, c, d and 72a, b, c, d; while the average frequency of these waves is a function of the dimensions of the cavities 69.

The manner in which the aforedescribed arrangement of coupling holes provides improved frequency-phase characteristics for the slow-wave structure 12 will now be discussed with reference to FIG. 4. In this figure the dashed curve A illustrates the frequency w as a function of the phase constant for electromagnetic waves propagating along a coupled coaxial cavity slow-wave circuit of the type illustrated in FIG. 1 except having only the outer coupling holes 79a, b, c, d. The phase velocity of the space charge wave in the electron beam used to interact with the electromagnetic slow-wave circuit wave is represented by the dashed line B extending from the origin through a portion of the curve A. The region of substantial coincidence between the curves A and B represents frequencies at which interaction between the electron `beam and the slow-wave circuit wave can occur, and thus, the generally greater range of coincidence between the curves A and B, the Wider the operating bandwidth of the slow-wave circuit.

Inspection of the curves A and B will reveal that even though the range of coincidence of these curves is large (so as to provide a wide bandwidth), the curves A and B are also quite close to one another near the upper cutoff frequency ou of the slow-wave circuit where the slope of the curve A approaches zero. Sincethe interaction impedance is high at this point, the possibility of oscillations is great. However, if the phase velocity of the electron beam space charge wave is altered (eg. by changing the'potential at which the cathode 16 is maintained) to that shown by the line B', it will be observed that the curves A and B are farther apart at the upper cutoff frequeniy wu, thereby reducing the tendency for oscillations. However, the curves A and B coincidev over a very small frequency range, and thus the operating bandwidth would be small.

By employing the arrangement of inner and outer coupling holes 72a, b, c, d and 76a, b, c, d, respectively, in the manner of the present invention, as shown in FIGS. 1-3 the w-,B characteristic of the slow-wave circuit wave assumes the shape illustrated by the ksolid curve A of FIG. 4. It will be observed that this curve is less dispersive in the central regions of the frequency passband of the slow-wave circuit and is more dispersive, i.e., undergoes a sharper bend away from the electronl beam phase velocity line B', in the vicinity of the upper cutoff frequency wu than the curve A. This provides a reasonably wide slowwave circuit passband, and at the same time, considerably v reduces the tendency for oscillation at the upper cutoff frequency wu. In addition, the symmetrical layout and relatively large number of coupling holes result in essentially uniform perturbation of the electric elds in the annular interaction region 74, thereby enhancing uniform interaction with all segments of the hollow electron beam.

Although the present invention has been shown and described with reference to a particular embodiment, nevertheless, various changes and modifications obvious to one skilled in the art are deemed to be within the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

l. A traveling-wave tube comprising: means for launching an annular stream of electrons along a predetermined path of xed length; an electrically conductive tubular envelope concentrically disposed about and axially aligned with said path; an electrically conductive rod concentrically disposed within and axially aligned with said path; a plurality of first annular electrically conductive plates aixcd to said envelope at spaced points therealong and extending radially inwardly therefrom in planes perpendicular to said path, each of said first annular plates delining in its innermost region a irst annular rim portion extending outwardly from said plane of said iirst plate in a direction parallel to said path; a plurality of second annular electrically conductive plates affixed to said rod and each extending radially outwardly therefrom in the plane of one of said rst annular plates, each said second annular plate dening in its outermost region a second annular rim portion extending outwardly from said plane in a direction parallel to said path, said second annular rims being coextensive with said i'irst annular rims; said envelope, said first and second annular plates and said rod defining a plurality of cavities; preselected ones of said iirst annular plates each dening four substantially kidneyshaped coupling apertures, the centers of which are spaced 90 apart circumferentially along said iirst annular plate; each of said second annular plates coplanar with one of said preselected first annular plates delining four substantially kidney-shaped coupling apertures smaller than the coupling apertures in said first annular plates; the centers of said coupling apertures in each of said second annular plates being spaced 90 apart circumferentially along said second annular plate and being angularly displaced by 45 with respect to the centers of said coupling apertures in the rst annular plate coplanar with said second annular plate; and the centers of the coupling apertures in the iirst annular plate at one end of at least one of said cavities being angularly displaced `by 45 with respect to the centers of the coupling apertures in said iirst annular plate at the opposite end of said one cavity.

2. A traveling-Wave tube comprising: means for launching a stream of electrons along a predetermined annular path, an electrically conductive tubular envelope concentrically disposed about and axially aligned with said annular path, an electrically conductive rod concentrically disposed within and axially aligned with said annular path, a plurality of electrically conductive wall means mounted in said envelope at spaced points therealong, said Wall means extending perpendicular to said envelope and to said rod to define therewith a plurality of annular cavities, said wall means defining aligned annular apertures to provide a passage for said electron stream, preselected ones of said Wall means each further delining a plurality of outer coupling apertures equally spaced circumferentially in regions radially outwardly of its said annular aperture and a like plurality of inner coupling apertures equally spaced circumferentially in regions radially inwardly of its said annular aperture, each outer coupling aperture in each said preselected wall means being angularly displaced with respect to each outer coupling aperture in each axially adjacent wall means, each inner coupling aperture in each said preselected Wall means being angularly displaced with respect to each inner coupling aperture in each axially adjacent Wall means, and each inner coupling aperture in each preselected Wall means being angularly displaced with respect to each outer coupling aperture in the same Wall means with the center of each inner coupling aperture lying angularly midway between the centers of the two nearest outer coupling apertures.

3. A traveling-wave tube comprising: mean-s for launching a stream of electrons along a predetermined annular path, an electrically conductive tubular envelope concentrically disposed about and axially aligned with said annular path, an electrically conductive rod concentrically disposed within and axially aligned with said annular path, a plurality of electrically conductive wall means mounted in said envelope at spaced points therealong, said wall means extending perpendicular to said envelope and to said rod to dene therewith a plurality of annular cavities, said wall means delining aligned annular apertures to provide a passage for said electron stream, preselected ones of said wall means each further defining a plurality of outer coupling apertures equally spaced circumferentially in regions radially outwardly of its said annular aperture and a like plurality of inner coupling apertures equally spaced circumferentially in regions radially inwardly of its said annular aperture, each inner coupling aperture in each preselected wall means being angularly displaced with respect to each outer coupling aperture in the same wall means, each outer coupling aperture in the wall means at one end of at least one of said cavities being angularly aligned with a different inner coupling aperture in the wall means at the other end of said one cavity, and each inner coupling aperture in said Wall means at said one end of said one cavity being angularly aligned with a diferent outer coupling aperture in said wall means at said other end of said one cavity.

References Cited in the ile of this patent UNITED STATES PATENTS 2,636,948 Pierce Apr. 2s, 1953 2,637,001 Pierce Apr. 28, 1953 2,899,598 Ginzton Aug. 11, 1959 2,943,229 Branch et al June 28, 1960 2,952,795 Craig et al Sept. 13, 1960 3,010,047 Bates Nov. 21, 1961 3,114,072 Belohoubek Dec. 10, 1963 FOREIGN PATENTS 850,521 Great Britain Oct. 5, 1960 

2. A TRAVELING-WAVE TUBE COMPRISING: MEANS FOR LAUNCHING A STREAM OF ELECTRONS ALONG A PREDETERMINED ANNULAR PATH, AN ELECTRICALLY CONDUCTIVE TUBULAR ENVELOPE CONCENTRICALLY DISPOSED ABOUT AND AXIALLY ALIGNED WITH SAID ANNULAR PATH, AN ELECTRICALLY CONDUCTIVE ROD CONCENTRICALLY DISPOSED WITHIN AND AXIALLY ALIGNED WITH SAID ANNULAR PATH, A PLURALITY OF ELECTRICALLY CONDUCTIVE WALL MEANS MOUNTED IN SAID ENVELOPE AT SPACED POINTS THEREALONG, SAID WALL MEANS EXTENDING PERPENDICULAR TO SAID ENVELOPE AND TO SAID ROD TO DEFINE THEREWITH A PLURALITY OF ANNULAR CAVITIES, SAID WALL MEANS DEFINING ALIGNED ANNULAR APERTURES TO PROVIDE A PASSAGE FOR SAID ELECTRON STREAM, PRESELECTED ONES OF SAID WALL MEANS EACH FURTHER DEFINING A PLURALITY OF OUTER COUPLING APERTURES EQUALLY SPACED CIRCUMFERENTIALLY IN REGIONS RADIALLY OUTWARDLY OF ITS SAID ANNULAR APERTURE AND A LIKE PLURALITY OF INNER COUPLING APERTURES EQUALLY SPACED CIRCUMFERENTIALLY IN REGIONS RADIALLY INWARDLY OF ITS SAID ANNULAR APERTURE, EACH OUTER COUPLING APERTURE IN EACH SAID PRESELECTED WALL MEANS BEING ANGULARLY DISPLACED WITH RESPECT TO EACH OUTER COUPLING APERTURE IN EACH AXIALLY ADJACENT WALL MEANS, EACH INNER COUPLING APERTURE IN EACH SAID PRESELECTED WALL MEANS BEING ANGULARLY DISPLACED WITH RESPECT TO EACH INNER COUPLING APERTURE IN EACH AXIALLY ADJACENT WALL MEANS, AND EACH INNER COUPLING APERTURE IN EACH PRESELECTED WALL MEANS BEING ANGULARLY DISPLACED WITH RESPECT TO EACH OUTER COUPLING APERTURE IN THE SAME WALL MEANS WITH THE CENTER OF EACH INNER COUPLING APERTURE LYING ANGULARLY MIDWAY BETWEEN THE CENTERS OF THE TWO NEAREST OUTER COUPLING APERTURES. 